Nature of the lowest triplet states of 4 '-substituted N-phenylphenothiazine derivatives

The nature of the lowest triplet state of the donor-acceptor N-phenylphenot hiazine derivatives has been studied by means of phosphorescence and EPR sp ectroscopy in various low temperature glasses. The triplet excitation of N- phenylphenothiazine and N-(p-anisyl)-phenothiazine is localised within the phenothiazine subunit. On the contrary, the 77 K phosphorescence of the mol ecules containing an electron acceptor group (i.e. -CN, -COCH3 or -COC6H5 a t the 4' position) is similar to that for benzonitrile, acetophenone or ben zophenone, respectively, although the energy levels of their T-1 states are higher than that of phenothiazine. With increasing temperature, however, t heir phosphorescence becomes similar to that characteristic for phenothiazi ne. This finding has been explained in terms of the excited-state intramole cular energy transfer accompanied by the planarisation of the phenothiazine kernel. The results of crystallographic investigations support this hypoth esis.